Search

Multiple myeloma (MM) is an incurable hematological malignancy characterized by remission and relapse with drugs resistance. Therefore, new therapeutic approaches are needed. Oncolytic viruses (OVs) represent a new strategy to augment the spectrum of cancer therapeutics. Several studies reported that, in MM, the OVs act through tumor-specific oncolysis and generation of an antitumor immune response. The main viruses that have been tested for MM setting are human viruses, this approach is highly restricted by pre-existing anti-virus humoral immunity that neutralize the anti-tumor effect of OVs. Recently, our group have demonstrated for the first time the role of a bovine virus, non-pathogen for the human, Bovine Viral Diarrhea Virus (BVDV) in direct MM cell killing suggesting its possible use as alternative strategy in MM oncolytic virotherapy. The aim of this study was to increase the bovine oncolytic viruses’s spectrum for anti-MM treatment investigating the role of another bovine virus, Bovine Herpes Virus type 1 (BoHV-1), in the direct effect on MM cells. Firstly, after virus preparation, we treated human MM cell lines (HMCLs) for 24, 48 and 72 hours with BoHV-1 at 1 and 2 multiplicity of infection (MOI). We showed a significant increase of cell mortality, checked by flow cytometry analysis, already after 48 hours of infection in JJN3 (BoHV-1 1 MOI vs untreated p< 0.001; BoHV-1 2 MOI vs untreated p< 0.001) and in MM1.S (BoHV-1 1 MOI vs untreated p= 0.025; BoHV-1 2 MOI vs untreated p= 0.002). Interestingly, the cytotoxic effect of BoHV-1 treatment in HMCLs was associated by a significant increased expression of apoptotic markers as Apo2.7, evaluated by flow cytometry (JJN3 at 48 hours: BoHV-1 1 MOI vs untreated p= 0.007; BoHV-1 2 MOI vs untreated p= 0.001. MM1.S at 48 hours: BoHV-1 1 MOI vs untreated p< 0.001; BoHV-1 2 MOI vs untreated p< 0.001). Subsequently, we infected bone marrow mononuclear cells (BMMNCs) obtained from MM patients with BohV-1 for 48 and 72 hours. Already after 48 hours of infection, we found that MM cells had significantly increased cell mortality in BoHV-1-treated BMMNCs compared with untreated conditions. Overall, our data indicate that BoHV-1, as BVDV, was able to exert a direct anti-tumor effect on both HMCLs and primary cells from MM patients. In addition, data from ongoing studies will characterize the role of immune microenvironment in bovine oncolytic virotherapy. Focusing on monocytes, NK cells and CD8+ T cells we will explore a possible bovine virus-induced immune response to enhance anti-MM virotherapy. This study will highlight the possible use of non-human OVs as new anti-MM strategy. Citation Format: Valentina Marchica, Rosanna Vescovini, Valentina Franceschi, Paola Storti, Nicolas Thomas Iannozzi, Vincenzo Raimondi, Oxana Longu, Jessica Burroughs Garcia, Denise Toscani, Anna Benedetta Dalla Palma, Gaetano Donofrio, Nicola Giuliani. Oncolytic virotherapy with non-human viruses to improve anti-myeloma effect. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 6797.

The amplification of the 1q21 (amp1q21) region is one of the most acquired cytogenetic abnormalities (CA) in multiple myeloma (MM) associated with a worse patient outcome and disease progression. Moreover, different studies have demonstrated that the number of copies (CN) 1q21 (gain1q21: three copies or amp1q21: ≥ four copies) have a different impact in the response to anti-MM therapies. Particularly, it has been proposed that in MM patients, additional copies of 1q21 may be associated with the resistance to proteasome inhibitor (PI) treatment as bortezomib. A recent study showed that newly diagnosed MM (MMD) patients carrying amp1q21 but not gain1q21 receiving carfilzomib-based treatment have an early disease progression with shorter overall survival. Previous studies underlined that the amplification of 1q21 can lead to the overexpression and/or dysregulation of several candidate genes associated with cell proliferation, apoptosis, and drug resistance. Here we aim to identify 1q21 target genes possibly correlated to the response to PI therapy. We evaluated a total cohort of 29 primary plasma cells (PCs) purified from bone marrow (BM) blood aspirates from 11 smoldering MM (SMM) and 18 MMD. The median age of our cohort was 70 years (range: 38-86). Fluorescence in situ hybridization (FISH) analysis was performed to access the presence or absence of copy number alteration (CNA) in the 1q21 region in all patients. 14 out of 29 patients carried 1q21 CNA (5 with gain1q21 and 9 with amp1q21). A score reflecting the number of 1q21 copies was calculated based on the hybridization pattern. The transcriptional profiles of the 29 BM PCs samples were generated on GeneChip ClariomD Arrays (Affymetrix Inc., Santa Clara, CA, USA). The samr package was used in R for call genes as differentially expressed between 1q21 CN-altered and wild-type samples. The correlation between the 1q21 copy number score and the gene expression levels was performed. Moreover, we have evaluated by FISH the 1q21 CNA in a panel of human myeloma cell lines (HMCLs): OCY-MY5, JJN3, RPMI-8226, NCI-H929, and OPM2. JJN3 were transfected with a control vector and PSMB4 and PSMD4 short hairpin RNA (shRNA) lentivectors. The gene and protein expression levels of PSMB4 and PSMD4 in MM cell lines were analyzed by qRT-PCR and Western Blot, respectively. Cell viability and proliferation were assessed using MTT assay and flow cytometry. Our bioinformatic analyses highlighted the overexpression of different genes (IL6R, ILF2, BCL9, MCL1, CSk1B, ADAR1, ARNT, ANP32E) in the 1q21 CNA samples with respect to the controls, as already reported in the literature. Our analysis showed a significantly higher expression of two proteasome subunits (PSMB4 and PSMD4) in patients with 1q21 CNA when compared with patients without (PSMB4 p=0.0006; PSMD4 p= We have evaluated PSMB4 and PSMD4 mRNA and protein expression levels in a 1q21 wild-type cell line (OCY-MY5) and in a panel of MM cell lines carrying different degrees of 1q21 CN (in order: JJN3, U266, RPMI-8226, OPM2, and NCI-H929). The mRNA expression level of PSMB4 and PSMD4 was higher in cell lines carrying 1q21 amp, following a 1q21 copy number fashion. Similar results were obtained when protein levels of MM cell lines were analyzed by Western Blot. To further determine the potential role of both proteasome subunits in the pathogenesis of amp1q21, we generated a PSMB4-shRNA and PSMD4-shRNA knockdown stable MM cell lines. Functional studies showed that blockade of PSMB4 and PSMD4 decreased MM cell viability. In conclusion, our study identified proteasome subunits PSMB4 and PSMD4 to be significantly upregulated in MM patients carrying amp1q21, correlated with 1q21 copy number but not with disease stage. In addition, knockdown of both, PSMB4 and PSMD4 decreased MM cell proliferation. Therefore, targeting PSMB4 and PSMD4 could be a strategy to treat MM patients with ampq21 Disclosures Giuliani: Celgene: Membership on an entity's Board of Directors or advisory committees, Other: congress, Research Funding; Bristol Mayers Squibb: Other: congress; GSK: Other: clinical studies; Takeda: Membership on an entity's Board of Directors or advisory committees; Janssen: Membership on an entity's Board of Directors or advisory committees, Other: Clinical studies, congress, Research Funding; Millenium Pharmaceutical: Other: clincial studies.

Despite the promising results of immune-checkpoint blockade in the treatment of many tumors, the use anti-PDL-1/PD-1 antibodies in multiple myeloma (MM) still remains debated and under observation for the high toxicity in combination with immunomodulatory drugs or for the lack of a clear evaluation of the PD-L1/PD-1 distribution in MM patients. Literature data on PD-L1/PD-1 expression by CD138+ and bone marrow (BM) cells in MM patients are discordant and none of them compared patients with active and smoldering myeloma (SMM). This suggests the need to better define PD-L1/PD-1 distribution in the BM immune-microenvironment in MM patients, to identify those that could benefit from PD-L1/PD-1 blockade. In this study, we isolated mononuclear cells from BM aspirates in a cohort of patients with monoclonal gammopathies (total number= 107) including 39 patients with SMM and 78 with active MM, including both newly diagnosed (MMD) and relapsed MM (MMR). We compared the expression profile of PD-L1/PD-1 axis on CD138+ cells, CD14+ monocytes and T cells (both CD4+ and CD8+), by flow-cytometry. Results were correlated with clinical parameters, as International Staging system (ISS), cytogenetic risk, bone disease. BM sera were also collected to measure the levels of different soluble factors known to regulate PD-L1 expression or to exert pro/anti-tumor activity in MM (IL-6, IL-10, IL-27, IFN-γ). Results from ELISA assay were examined in relation with flow-cytometry data. We found that neither PD-L1 expression on CD138+ cells nor PD-1 on CD4+/CD8+ cells significantly differ between SMM and MM patients; although, CD14+PD-L1+% increases with disease progression (SMM vs MMD vs MMR: median: 39.14 vs 59.49 vs 47.66, p=0.14) without reaching a statistical significance. Analysis on the total cohort revealed that PD-L1 is expressed at higher levels on CD14+CD16+ non-classical monocytes compared with classical monocytes (17.41 vs 23.09, p Focusing on patients with active MM, those with ISS=II and III showed increased PD-L1 expression on CD14+ cells (ISS II+III vs I, median MFI 20.35vs14.59, p=0.005) and higher CD8+PD-1+% (II+III vs I, 4.35vs2.58, p=0.022) compared with ISS=I patients. Analysis on PD-L1/PD-1 expression in relation with the cytogenetic features of our cohort of patients revealed that CD138+ cells from hyperdiploid patients express higher levels of PD-L1 compared with not-hyperdiploid ones (25.66 vs 13.96, p=0.001), suggesting that the expression of this immune checkpoint does not contribute as a high risk factor in MM disease. Finally, we investigated PD-L1/PD-1 distribution, in relation with the presence of bone lesions, and we found that not-osteolytic patients have higher CD8+PD-1+% compared with osteolytic ones (p=0.071). In conclusion, our data show a similar PD-L1/PD-1 expression pattern between SMM and MM patients, and higher PD-L1 intensity on the non-classical monocytes CD14+CD16+ compared with classical CD14+CD16- cells. On the other hand, an inverted CD4+/CD8+ ratio characterizes relapsed MM patients, together with high amount of MM pro-survival IL-6 and low anti- tumor IL-27 BM serum levels. Overall these data suggest that, despite the similarity in PD-L1/PD-1 expression, SMM and early MM patients rather than relapsed MM, could represent the potential subset which could better benefit of anti PD-L1/PD-1 therapy, in light of their less compromised immune-microenvironment. Disclosures Giuliani: Celgene: Membership on an entity's Board of Directors or advisory committees, Other: Participation in congresses, Research Funding; Takeda: Membership on an entity's Board of Directors or advisory committees; Bristol-Myers Squibb: Other: Participation in congresses; GSK: Other: Clinical study sponsorship, Research Funding; Janssen Pharmaceutical: Membership on an entity's Board of Directors or advisory committees, Other: Clinical study sponsorship; participation in congresses, Research Funding; Millennium Pharmaceutical: Other: Clinical study sponsorship, Research Funding.

The identification of risk factors for progression is critical in the clinical management and appropriate follow up of patients with pre-malignant Asymptomatic Monoclonal Gammopathies (AMG) including Monoclonal Gammopathy of Undetermined Significance (MGUS) and Smoldering Multiple Myeloma (SMM). The development of prognostic score and consequently the early identification of patients with possible short-term progression to Multiple Myeloma (MM) could lead to anticipate the treatment. In this study, we retrospectively evaluated possible risk factors of short-term progression to active MM in a large cohort of MGUS and SMM patients admitted to a single haematological center (Hematology and BMT Unit, University Hospital of Parma) between 2010 and 2018. We analysed a total cohort of 235 patients diagnosed with AMG (81 MGUS and 154 SMM) according to the IMWG recently updated diagnostic criteria. All patients analysed underwent to Bone Marrow (BM) examination; moreover, imaging evaluation was performed in 22 MGUS and 123 SMM patients, in order to exclude the presence of bone disease. In a subgroup of AMG patients (n=50), bone mineral density (BMD) evaluation by Dual-energy X-ray Absorptiometry (DXA) was also available. Median age of the AMG patients analysed was 68 years (range 35-93 years). Median percentage of BM plasma cells (BMPCs) was 12% (range 2-55%) in the entire population, 7% (range 2-9) in MGUS and 15% (range 10-55) in SMM patients. Median serum M-protein was 1.7 g/dL (range: 0.17-4.5), 1.5 g/dL (range 0.17-4.5) in MGUS and 1.8 g/dL (range 0.4-2.7) in SMM patients. An abnormal free light chain (FLC) ratio was found in 70% of AMG patients, among the ones that performed the analysis; regarding SMM patients, FLC ratio value was available in 97 patients: in 72 (76%) the ratio was unbalanced, 37 (39%) had a FLC ratio ≤ 0.125 or ≥ 8 and in 14 (15%) it was > 20; among MGUS patients, value was collected in 41 patients and in 21 (51%) it was 1.65. The presence of immunoparesis in one or two uninvolved immunoglobulins occurred in 59% of the entire population. The median follow up time was 18 months (range 0 - 111 months) for whole population. Overall 44 patients of the entire cohort progressed to MM (41 SMM and 3 MGUS) with a median TTP of 14.5 months. By univariate analysis we found that percentage of BMPCs, entity of M-protein and presence of immunoparesis were significantly correlated with progression to active MM (p 8 (as used in Mayo scoring system for SMM) showed a relationship at the limit of statistical significance in this subgroup of patients (p=0.052). Any significant correlation was not observed with age, sex, Ig isotype, light chain's type and the BMD values (p=NS). Afterwards, we applied Kaplan Meier method on risk factors resulted significant in univariate analysis demonstrating that they also significantly influenced the time to progression to MM. Finally, through a binomial logistic regression, we developed a new prognostic score for whole population. By combining the values of M-protein (< 2, score=0 or ≥ 2 g/dL, score=1) and the percentage of BMPC (20%, score=2), we obtained six groups at different probability of progression to active MM (Table 1). Given that result, we stratified patients in 3 groups: low-risk (score=0), intermediate-risk (score=1) and high-risk (score≥2); log-rank test confirmed that high-risk patients had a significantly shorter time to progression to symptomatic MM as compared to intermediate and low-risk patients (p In conclusion, our results show that in patients with AMG the clinical factors, which mostly impact on the short-term risk of progression to active MM, are the entity of the PCs infiltrate and the MC related to the tumoral mass. The development of a clinical score based on BMPCs and M-protein will permit to overcome the traditional distinction between MGUS and SMM in the evaluation of the progression of AMG patients to active MM. Disclosures Giuliani: Janssen: Research Funding.

Alterations of the bone marrow (BM) immune-microenvironment characterize the progression of monoclonal gammopathies and the development of osteolytic bone disease in multiple myeloma (MM). MM patients exhibit immune dysfunctions as impaired dendritic, NK and T cells, whereas the onset of MM osteolytic lesions is associated to an increased prevalence of Th17 cells. Recently, the pathophysiological role of the programmed cell death protein 1 (PD-1)/PD-1 ligand (PD-L1) pathway together with an increase of myeloid derived suppressor cells (MDSCs) in the induction of tumor tolerance and immune evasion has been underlined with a therapeutic relevance. However, unclear data on the expression profile of PD-1/PD-L1 axis in MM patients have been reported and it is not known if this axis could be related with the presence of osteolytic bone disease. In order to elucidate these aspects, we analyzed a total cohort of 80 consecutives patients with monoclonal gammopathies, including 15 monoclonal gammopathy of undetermined significance (MGUS), 23 smoldering MM (SMM), 21 newly diagnosed MM (MMD) and 21 relapsed/refractory MM (MMR) patients. The presence of bone disease was checked by low-dose computerized tomography (CT) with or without positron emission tomography (PET) scan and by X-rays skeletal survey in 11 MM patients. 87% of MM patients displayed osteolytic lesions. High bone disease (HBD) was defined by the presence of 3 or more osteolytic lesions (62% of our cohort). Patients without bone lesions or with minus of 3 lesions were defined as low bone disease (LBD). BM mononuclear cells were analyzed by flow cytometry, evaluating plasma cells (PCs) (CD138+), monocytes (CD14+) and T cells (total CD3+, CD3+CD4+ and CD3+CD8+). PD-L1 (CD274) expression was evaluated on CD138+ and CD14+ cells, and PD-1 (CD279) on CD3+, CD4+ and CD8+ cells. Lastly, in a subgroup of patients we analysed MDSC populations, including both granulocytic (gMDSCs) (CD11b+HLA-DR-CD14-CD15+) and monocytic MDSCs (mMDSCs), (CD11b+HLA-DR-/lowCD14+CD15-). The percentage of BM CD3+PD-1+ cells increased from MGUS to MMR patients with a significant trend (p=0.004). Indeed, patients with active MM (MMD and MMR) showed both higher % of CD3+PD-1+ cells (median value: 48.5% vs 40.6%, p=0.001) and PD-1 median fluorescence intensity (MFI) on CD3+ (median value: 18.9 vs 16.7 MFI, p=0.024) as compared to patients with SMM and MGUS. CD4+PD-1+, but not CD8+PD-1+ cells are increased in active MM compared to SMM and MGUS patients (p=0.023). On the other hand, any significant difference was not observed in the PD-L1 expression on both BM CD138+ and CD14+ cells across patient groups. The percentage of BM MDSC populations did not significantly change across the different monoclonal gammopathies (p=0.14); moreover, comparing MM with SMM and MGUS patients, the % of gMDSCs was significantly reduced (median %: 12.5% vs 17%, p=0.04) and the % of mMDSCs was increased (median %: 0.36% vs. 0.24%) without reaching a statistical significance. Focusing on MM bone disease, we found that osteolytic MM patients displayed higher CD4+/CD8+ ratio compared to non-osteolytic ones (p=0.043). Regarding the PD-1 expression, the % of CD3+PD-1+ cells did not change (p=0.192), whereas the % of CD8+PD-1+ cells was significantly reduced in osteolytic patients compared to non-osteolytic ones (p=0.016). Consistently, among the BM CD8+ cells, a significant decrease of %PD-1+ cells was found in HBD vs LBD MM patients (median value: 46.9% vs 57.4%, p=0.045). Interestingly, when compared to LBD MM patients, HBD MM patients displayed a significant reduction of PD-L1 expression on both BM CD138+ PCs (median MFI value: 13.3 vs 21.6 MFI, p=0.007) and CD14+ cells (median MFI value: 15.4 vs 23.8 MFI, p=0.007). In a multivariate analysis, PD-1+ expression on CD8+ cells and PD-L1 MFI on CD138+ were significant independent factors related to the presence of HBD. In conclusion, our study indicates that the frequency of PD-1+ T cells increases across the progression of the monoclonal gammopathies. On the other hand, for the first time, we show in MM patients a significant relationship between the presence of extensive osteolytic bone disease and a reduced expression profile of BM PD-1/PD-L1 axis on CD8+ and CD138+ cells. We hypothesize that a less immune-suppressive profile could be related to the development of osteolysis consistent with the negative cross talk existing between PD-1/PD-L1 axis and Th17 cells. Disclosures Aversa: Pfizer: Honoraria, Membership on an entity's Board of Directors or advisory committees; Astellas: Honoraria; Basilea: Honoraria, Membership on an entity's Board of Directors or advisory committees; Merck: Honoraria; Gilead: Honoraria, Membership on an entity's Board of Directors or advisory committees. Giuliani:Takeda Pharmaceutical Co: Research Funding; Janssen Pharmaceutica: Other: Avisory Board, Research Funding; Celgene Italy: Other: Avisory Board, Research Funding.

Multiple myeloma (MM) cells are characterized by tight dependence on the bone marrow (BM) microenvironment that exerts a permissive role on cell growth and survival. In turn, MM cells markedly modify their microenvironment leading, in particular, to the development of osteolytic bone lesions. Recently, we demonstrated that metabolic alterations is a major feature of MM cells showing that BM plasma of MM patients is characterized by lower levels of Glutamine (Gln) and higher levels of Glutamate (Glu) and ammonium when compared with patients with smoldering MM (SMM) and Monoclonal Gammopathy of Uncertain Significance (MGUS). In the majority of MM patients MM cells are Gln-addicted since they strictly depend on extracellular Gln, do not express Glutamine Synthetase (GS), the enzyme that synthetizes Gln from Glu and ammonium, and are endowed with high levels of the Gln transporter ASCT2. Based on this evidence, we have hypothesized that the peculiar Gln metabolism of MM cells may have a significant impact on the relationship with the bone microenvironment and contribute to the development of osteolytic lesions. We firstly characterized a panel of human MM cell lines (HMCLs) for their GS expression and response to decreasing levels of Gln. The majority of HMCLs, which did not express GS, consumed large amounts of extracellular Gln but secreted nearly half of the amino acid as Glu. Two HMCLs, MM1.S and U266, with a sizable GS expression, were less sensitive to Gln deprivation and secreted less Glu in the extracellular space compared with GS-negative HMCLs. Consistently, the activity of the Glu exchanger x-CT (the product of SLC7A11 gene) was lower in GS-positive than in GS-negative cells. The response to Gln starvation was then studied in mesenchymal stromal cell line (MSC), as well as in osteoblastic (HOBIT) and pre-osteocytic cells (HOB-01). HOBIT and HOB-01 were more sensitive to Gln depletion than MSC. Indeed, while MSC showed a low EC50 for Gln (0.064mM), which is 10-times lower than the physiological blood Gln concentration (around 0.6 mM), the EC50 values of HOBIT and HOB-01 cells were 0.250 mM and 0.297mM, respectively. Furthermore, L-methionine sulfoximine (MSO), an irreversible inhibitor of GS, emphasized the effects of Gln deprivation on all the cell lines tested. Indeed, Gln deprivation enhanced the expression of GS, suggesting that both stromal and osteoblastic cells exploit the enzyme to counteract Gln deprivation. On the basis of these data, we assessed the effects of Gln and Glu on osteogenic differentiation by incubating MSC, either immortalized or primary, with an osteogenic medium containing different concentrations of Gln and Glu. After 2 weeks, compared with cells differentiated in high Gln/high Glu conditions, MSC incubated in the presence of decreased Gln and increased Glu showed lower osteogenic ability, as assessed by real time PCR and ALP staining. Lastly, MSC co-cultured for 72 hours with GS-negative, but not with GS-positive HMCLs, showed reduced viability and increased GS expression. Lastly, to put in a translational perspective these in vitro observations, we analyzed the BM plasma levels of Gln and Glu in a cohort of 41 patients with newly diagnosed MM, including 9 smoldering MM (SMM) and 32 active MM patients (20 of them with osteolytic bone disease, 12 of them without bone disease). All 20 osteolytic MM patients had more than three osteolytic lesions. We found that MM patients had lower Gln levels and higher Glu levels than SMM patients. Moreover, when compared with MM patients without bone disease, MM patients with bone disease showed lower levels of Gln and higher levels of Glu. The results of these analyses are being continuously updated increasing the number of samples tested. Overall, these results indicate that MM cells are able to create a low-Gln/high-Glu bone marrow microenvironment that sustains GS expression in bone cells and impairs their differentiation and viability. Thus, the peculiar metabolic milieu in the MM bone microenvironment affects the relationship between neoplastic and bone cells and may contribute to the development of osteolytic bone disease in MM patients. Disclosures Aversa: Astellas: Honoraria; Merck: Honoraria; Pfizer: Honoraria, Membership on an entity's Board of Directors or advisory committees; Basilea: Honoraria, Membership on an entity's Board of Directors or advisory committees; Gilead: Honoraria, Membership on an entity's Board of Directors or advisory committees. Giuliani:Takeda Pharmaceutical Co: Research Funding; Celgene Italy: Other: Avisory Board, Research Funding; Janssen Pharmaceutica: Other: Avisory Board, Research Funding.

Recently, the introduction of anti-CD38 monoclonal antibody, Daratumumab (DARA), in multiple myeloma (MM) therapy has improved the response rate of relapsed MM patients. However only a fraction of the DARA-treated patients respond, thus further studies on DARA mechanisms of action are needed. Because the antibody dependent cellular phagocytosis (ADCP) mediated by monocyte, is one of the mechanisms through DARA exerts its anti-MM activity, an ex-vivo approach was established in order to investigate which mechanisms or patient's immunological characteristics could influence DARA-mediated killing of MM cells. Bone marrow mononuclear cells (BM-MNCs) obtained from 25 MM patients (12 newly diagnosed and 13 relapsed MM) were analyzed at time 0 (T0) by flow cytometry. We checked the % of plasma cells (PCs) (CD138+ cells), % of total monocytes (CD14+ cells) and their two subsets (CD14+CD16- or CD14+CD16+ cells), the ratio between % of CD14+ and % of CD138+ cells (CD14+:CD138+ ratio) and the median fluorescence intensity (MFI) of CD38 and of the immuno-check points CD47 on PCs and CD172a (SIRPα) on monocytes. Subsequently, BM-MNCs were treated with control IgG (10µg/ml), DARA (10µg/ml) and the F(Ab)2 portion of DARA (DARA F(Ab)2) (10µg/ml) for 48 hours and then the % variation of surviving 7AAD- CD138+ cell, the modification of the % of monocyte or subsets analyzed and the % of PCs that are attached to monocyte (identified as CD138+ cells also positive to CD14) were evaluated by flow cytometry. Firstly, we found that DARA significantly exploited its anti-MM activity (median % variation of surviving CD138+ cells: 69.05%, p=0.0007) compared to IgG control while DARA F(Ab)2 did not have any killing effect (median % variation of surviving CD138+ cells: 97.33%) compared to the control. Secondly, we detected that the % of total of monocytes and their subsets (CD14+CD16+ or CD14+CD16-) composition were not modulated by DARA or DARA F(Ab)2 treatment compared to IgG. Indeed, only in presence of DARA, we observed that a double positive population of CD138+CD14+ cell significantly increased compared to IgG control (mean %: 15.76 vs 3.14, p=69.05% of surviving PCs). Interestingly, in the HR group compared to LR group were significantly increased the % of double positive CD138+CD14+ (median % HR:21.59 vs LR: 7.41, p=0.035), the % of CD138+ cells bonded to CD14+CD16+ (median % HR:13.28 vs LR: 3.93, p=0.048) and the CD14+:CD138+ ratio measured at T0 (median HR:0.63 vs LR: 0.32, p=0.0158). Moreover, in 5 relapsed MM patient, we have correlated the ex-vivo response to DARA with the type of in vivo response after DARA single-agent treatment. In addition in our cohort of patients, we observed that the % of surviving CD138+ cells after DARA treatment negatively correlate with the CD14+:CD138+ ratio at T0 (r:-0.628, p=0.0023), with the % of CD138+CD14+ population (r:-0.602, p=0.0039) and with the % of CD138+CD14+CD16+ population (r:- 0.657, p=0.0238) but did not correlate with the MFI of CD38 expression on PCs and the % of CD14+CD16+ at T0. Finally, to go further inside the mechanism involved in DARA response, we have explored the role of the inhibitory axis CD47-CD172a in the ADCP DARA-induced in 5 ex-vivo samples from our cohort of MM patients. We found that the MFI of CD47 strongly positively correlated with the % of surviving CD138+ cells (r: 0.9897; p=0.010) after DARA treatment; on the other hand, we have not reported any correlation with the MFI of CD172a on monocytes. The results of these analyses are continuously updating increasing the number of samples tested. In conclusion, these data highlight that monocyte binding to PCs, in particular those CD14+CD16+, plays a central role in DARA killing effect independently of % subset composition in the pre-treatment samples, suggesting that there are mechanisms that regulate the effectiveness of the monocyte-based killing effect on malignant PCs, as the immune-suppressive axis CD47-CD172a, giving the rationale design to identify new strategies to increase the efficiency of DARA-based therapeutic regimen. Disclosures Malavasi: Takeda Pharmaceutical Co: Membership on an entity's Board of Directors or advisory committees, Research Funding; Janssen Pharmaceutica: Membership on an entity's Board of Directors or advisory committees, Research Funding. Aversa:Gilead: Honoraria, Membership on an entity's Board of Directors or advisory committees; Astellas: Honoraria; Basilea: Honoraria, Membership on an entity's Board of Directors or advisory committees; Merck: Honoraria; Pfizer: Honoraria, Membership on an entity's Board of Directors or advisory committees. Giuliani:Takeda Pharmaceutical Co: Research Funding; Celgene Italy: Other: Avisory Board, Research Funding; Janssen Pharmaceutica: Other: Avisory Board, Research Funding.